Due to the widespread use of digital integrated circuits nowadays, it has become convenient to employ a logic element as the amplifier in a crystal oscillator circuit. Many such crystal oscillators are being introduced into service although the results are not always satisfactory, due to the non-linearity fo the active element used. Careful design can produce a reliable general purpose crystal oscillator for the frequency range 1 MHz to 100 MHz, whose active element is not necessarily linear over its full dynamic range. Such designs can also cover crystal oscillators requiring an extended tuning range. The basic requirements for crystal oscillator circuit design are the same no matter what circuit is used for the active element. In order that the oscillator may start when power is supplied to the circuit, and that it may deliver the required power to the load, the correct impedance, phase and gain relationships around the oscillator feedback path must be satisfied. That is, the crystal unit must be matched to the amplifier; the algebraic sum of the phase around the regenerative loop must be zero (criterion for positive feedback); and the amplifier's gain must be greater than unity at noise input levels.

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